Interpretive Summary: Drainage water disposal in California (USA) is limited because of environmental concerns related to high salt content and the presence of trace elements in drainage water (e.g. Se, B, As, Mo). Reducing drainage water volumes and salt mass for disposal is a necessary part of a long term solution that will allow irrigated agriculture to maintain its current productivity levels. In our 3-year field study we found that alfalfa yield was reduced approximately 30% as a result of using saline water for irrigation compared to irrigating with non-saline water. The overall alfalfa quality improved when irrigated with saline water compared to non-saline water. Both boron and chloride progressively accumulated in alfalfa hay during each growing season (when irrigated with saline water), but boron and chloride returned to previously low levels by the beginning the subsequent growing season due to the influence of sufficient winter rainfall and irrigating with low salinity water at the break of dormancy. There was measurable accumulation of salt in the soil profile and in the shallow groundwater as a result of using saline drainage water for irrigation. Farmers can reduce drainage water effluent by irrigating with saline drainage water and increase forage quality which can partially offset yield reductions. Winter rainfall and preseason irrigation are needed to maintain adequate salt leaching for log term stable production of alfalfa.

Technical Abstract:
A three year study investigated the use of saline (< 6 dS/m) drainage water for irrigation of salt tolerant alfalfa in the presence of shallow saline groundwater. The irrigation treatments included; irrigating twice between cuttings with non-saline water, 2) irrigating with moderately saline water twice between cuttings, and 3) once between cuttings with moderately saline water. Yield data demonstrated an approximately 30% reduction in total yield over 3 years as a result of using saline water for irrigation compared to irrigating with non-saline water. The overall alfalfa quality improved when irrigated with saline water compared to non-saline water. However, both boron and chloride progressively accumulated in alfalfa plant tissue during each growing season when irrigated with saline water. The plant tissue boron and chloride values returned to previously low levels by the start of the next growing season due to the influence of winter rainfall and irrigating with low salinity water at the break of dormancy. There was measurable accumulation of salt in the soil profile and in the shallow groundwater as a result of using saline drainage water for irrigation. A gradual decrease in depth to the shallow groundwater was observed as result of the lack of drainage capability. Electromagnetic induction proved adequate for characterizing root zone salinity with time.